As brought out in the above-mentioned copending application, numerous alarm systems have been developed for signaling a deleterious condition such as an overtemperature. In most cases the component of such a system that has caused the most difficulty and expense has been the sensor that responds to the condition to be detected and triggers operation of the device that emits the perceptible alarm signal.
A food freezer alarm sensor poses a particularly difficult problem. Such a sensor must respond to a rise in freezer cabinet temperature to above a predetermined value, but on the other hand it should set off the alarm only when an overtemperature condition has persisted for a significant length of time, instead of responding to merely transient temperature rises such as are due to opening of the freezer cabinet door or normal operation of the automatic defrosting cycle. A successful freezer alarm sensor which meets these requirements is disclosed in U.S. Pat. No. 4,011,552, to P. C. Quirke, but the sensor disclosed in that patent is relatively expensive to produce and its thermally responsive element responds with only a small change in current flow when temperature rises above the critical value, thus making necessary a small amplifier which further increases the cost of an alarm system incorporating that sensor.
The above identified copending application discloses a sensor which is adaptable to freezer alarm systems but which requires the preparation of a frozen pellet or tablet to render it suitable for such applications. Such preparation involves an element of substantial inconvenience that considerably offsets the low cost and reliability of the device.